This invention relates to electro-acoustic transducers and more particularly to an improved piezoelectric microphone suitable for use by divers at substantial depths.
Piezoelectric microphones have been utilized heretofore within diver's full face masks, helmets, and the like. One example is found in U.S. Pat. No. 3,562,451 issued to W. W. Mullen, Jr. The microphones disclosed therein utilize a piezoelectric bimorph in the form of a diaphragm that is waterproofed by being embedded in an elastomeric compound such as a silicone rubber material that is confined in a metal cup. Such microphone constructions have proven to be rugged, capable of withstanding water immersion under pressure, and to be conveniently small in size. While those microphones have served well at shallower depths, they suffer a marked degradation of sensitivity as depths and pressures increase, so much so that at depths of say 600 to 1000 feet, or more, they are not at all satisfactory.
Another disadvantage of piezoelectric microphones for diver use, is the high electrical impedance characteristics thereof that have required preamplifier devices to be carried by the diver in order to effectively couple the microphone with the remainder of the communication system.
Of course the problems of loss of sensitivity apply as well to piezoelectric transducers having similar construction and used as receivers or headphones.